This invention relates to an improved process for the production of polyphenylene ethers and more particularly to a precipitation process for the production of polyphenylene ethers from monohydric phenols by oxidative coupling.
Processes for producing polyphenylene ethers from 2,6-disubstituted phenols by oxidative coupling have long been known, including, for example, those disclosed in U.S. Pat. Nos. 3,306,874 and 3,219,626. More recently, the preparation of phenylene ether copolymers was disclosed in U.S. Pat. No. 4,011,200, and an improved catalyst system useful in the preparation of polyphenylene ethers was disclosed in U.S. Pat. No. 4,067,851.
Oxidative coupling polymerizations are generally carried out in the presence of a good solvent for the monomers and the polyphenylene ether and at concentrations that will result in a homogeneous polymer solution. It has long been known that these polymerizations could also be carried out in a medium which dissolves the monomer and catalyst but not the polyphenylene ether so that the high molecular weight polymer precipitates as it forms, as is shown in Example 27 of U.S. Pat. No. 3,306,875. As a practical matter however, polymerization in a good solvent had heretofore been thought necessary in order to afford means for removing catalyst residues and other impurities. According to a variety of references including U.S. Pat. No. 3,838,102, allowing the polyphenylene ether to precipitate in the presence of the catalyst results in a solid particulate polymer contaminated with catalyst residues which are very difficult to remove, with the result that the polymer exhibits instability and poor color. Further, as taught for example in U.S. Pat. No. 4,058,504, homogenious polymerization schemes allow better control of molecular weight in the final product.
The prior art has therefore tended to prefer processes for polyphenylene ether production that provide a homogeneous solution into which reaction-terminating compounds may be readily added to control final molecular weight and which may then be readily extracted with suitable immiscible compositions to remove the catalysts prior to precipitation of the polyphenylene ether. Although these homogeneous processes are quite successful commercially, there are disadvantages. It is necessary to avoid a highly viscous and difficult to handle reaction mixture by limiting the concentration of polymer in the final solution. Also, precipitation of the polymer from the homogeneous mixture after extraction of the catalyst residues presents a further practical problem. In general, precipitation is accomplished by mixing the polymer solution with a miscible non-solvent such as an alcohol, then filtering or centrifuging the mixture to recover the solid polymer. The solvent mixture cannot be re-used without a costly and technically undesirable fractional distillation to separate the solvent and precipitant.
Processes for a precipitation polymerization of monohydric phenols are also known in the art. For example, in U.S. Pat. No. 3,789,054, there was disclosed a two stage process in which the oxidative coupling of the phenols is carried out in a medium consisting of a solvent which will not dissolve the polymer after it attains a significant molecular weight (specific viscosity&gt;0.25). The solid particulate polymer produced in such processes is typically first separated from the media by centrifugation and/or filtration, then washed as a solid. Although no particular method is disclosed for washing the filtered polymer, the need for extensive washing to remove catalyst residues will be apparent from the teachings of the references previously discussed. The polymerization art to this time has not found a practical means to accomplish the necessary removal of catalyst residues from the precipitated polymer. For that reason, no commercially attractive precipitation polymerization scheme for the production of polyphenylene ethers is known.